In recent years, hybrid vehicles and plug-in hybrid vehicles, which use both an engine and an electric motor as driving sources for moving the vehicles, are gathering attention because they consume less fuel and emit less exhaust gases. For example, Japanese Unexamined Patent Application Publication No. 2010-30362 discloses a power transmission device for a vehicle mounted to a vehicle wheel, and including a driving force source such as an internal combustion engine, and an auxiliary power source such as an electric motor (motor-generator) capable of powering the vehicle and regeneration. While the vehicle is being powered by the driving force source, the auxiliary power source functions as an auxiliary driving source for the vehicle wheel, whereas the auxiliary power source generates regenerative energy during the regeneration mode, and this regenerative energy is stored in an energy storage device such as a battery.
Japanese Unexamined Patent Application Publication No. 2008-302892 discloses a driving system for a vehicle including a power transmission mechanism for dividing the driving force of an internal combustion engine, and transmitting the divided driving forces to a drive wheel and a generator. This power transmission device includes a ring gear as an output element arranged concentrically with a sun gear to which the rotor of a motor-generator is coupled, and is configured such that the sun gear and the ring gear rotate about their respective axes while revolving around the sun gear due to a carrier coupled to an engine output shaft. A permanent magnet motor is coupled to the ring gear, and provides additional driving torque. With this arrangement, it is possible to regenerate energy with the generator, and change the speed ratio in a stepless manner by changing the revolving speeds of the internal combustion engine and the motor.